1. The Field of the Invention
The present invention relates generally to a composite buoyancy module or can for supporting a object in water, like a riser of a floating oil platform or mooring lines. More particularly, the present invention relates to a buoyancy module formed of a composite outer shell or vessel, and a layer of buoyant material filling the volume of the shell or vessel.
2. The Background Art
For the purposes of describing the preferred embodiment, reference will be made to mainly one embodiment usage, that of an off shore platform riser support system. However, it is noted that there are many uses for the preferred embodiment that will become apparent to a skilled artisan after reviewing the specification, claims and drawings of the present invention. Specifically, the current invention easily can be applied to mooring lines used in the oil platform industry.
As the cost of oil increases and/or the supply of readily accessible oil reserves are depleted, less productive or more distant oil reserves are targeted, and oil producers are pushed to greater extremes to extract oil from the less productive oil reserves, or to reach the more distant oil reserves. Such distant oil reserves may be located below oceans, and oil producers have developed offshore drilling platforms in an effort to extend their reach to these oil reserves.
In addition, some oil reserves are located farther offshore, and thousands of feet below the surface of the oceans. Certain floating oil platforms, known as spars, or Deep Draft Caisson Vessels (DDCV) have been developed to reach these oil reserves. Steel tubes or pipes, known as risers, are suspended from these floating platforms, and extend the thousands of feet to reach the ocean floor, and the oil reserves beyond.
It will be appreciated that these risers, formed of thousands of feet of steel pipe, have a substantial weight which must be supported by buoyant elements at the top of the risers. Steel air cans have been developed which are coupled to the risers and disposed in the water to help buoy the risers, and eliminate the strain on the floating platform, or associated rigging. One disadvantage with the air cans is that they are formed of metal, and thus add considerable weight themselves. Thus, the metal air cans must support the weight of the risers and themselves. In addition, the air cans are often built to pressure vessel specifications, and are thus costly and time consuming to manufacture. The air cans are often pressurized with air to prevent water from filling the cans. Thus, another disadvantage with some air cans is the trouble associated with keeping the cans pressurized, such as air compressors, air lines, etc.
In addition, as risers have become longer by going deeper, their weight has increased substantially. One solution to this problem has been to simply add additional air cans to the riser so that several air cans are attached in series. It will be appreciated that the diameter of the air cans is limited to the width of the well bays within the platform structure, while the length is merely limited by the practicality of handling the air cans. For example, the length of the air cans is limited by the ability or height of the crane that must lift and position the air can. One disadvantage with more and/or larger air cans is that the additional cans or larger size adds more and more weight which also must be supported by the air cans, decreasing the air can""s ability to support the risers. Another disadvantage with merely stringing a number of air cans together is that long strings of air cans may present structural problems themselves. For example, a number of air cans pushing upwards on one another, or on a stem pipe, may cause the cans or stem pipe to buckle.
Another disadvantage of steel air cans is that buoyancy is lost if the air inside the air can is lost. The loss of enough buoyancy due to loss of air may cause the riser to collapse under its own weight. Substantially, the same problems exist for mooring lines and other devices needing to be floated. Steel or synthetic foam buoyancy elements using steel truss structural members are required to lift the weight of the mooring lines used to hold the platform in position. However, the buoyant elements are underwater and located at great distances from a compressed air source. Therefore, synthetic foams not requiring human intervention are used. Unfortunately, such foam fiber structures are difficult to make because of the structure""s own size makes tooling heavy and expensive. In addition, the resins used in syntactic foams undergo an exothermic reaction while curing. This heat must be released during curing or the foam will be damaged. The larger the part the more difficult it becomes to dissipate the heat.
Free standing riser systems, typically used in deep water oil and gas recovery, extends from the ocean floor to within 100 to 500 feet of the ocean surface. Below these depths, the riser is relatively free from the surface effects of wind, surface waves and currents. To maintain the free standing risers, air filled buoyancy elements get the top of the riser to provide the required tension to maintain the structure at the highest possible position. These air cans suffer from the same problems as air cans located on other oil recover platforms.
It has been recognized that it would be advantageous to optimize the systems and processes of accessing oil reserves, such as deep water oil reserves. In addition, it has been recognized that it would be advantageous to develop a system for reducing the weight of air cans, and thus the various riser systems and platforms. In addition, it has been recognized that it would be advantageous to develop a system for increasing the buoyancy of the air cans. In addition, it has been recognized that it would be advantageous to develop a system for providing buoyancy without the use of air pressure.
The invention provides a modular buoyancy system including one or more buoyancy modules. The buoyancy modules are vertically oriented, disposed at and below the surface of the water and coupled to a riser or stem pipe to support the riser. The one or more buoyancy modules are sized to have a volume to produce a buoyancy force at least as great as the riser or mooring lines to which they are attached, for example.
In accordance with one aspect of the present invention, the buoyancy module advantageously includes a composite vessel having a volume sized to produce a buoyancy force. The stem pipe is disposed concentrically through the composite vessel and receives the riser therethrough. A modular layer of buoyant material advantageously is disposed in the volume of the composite vessel, between the stem pipe and the composite vessel. Preferably, the volume is substantially filled by the layer of buoyant material, such that the layer of buoyant material substantially occupies the volume and prevents occupation of the volume by water. In addition, the layer of buoyant material may include a layer of foam material.
In accordance with another aspect of the present invention, the layer of buoyant material may include a plurality of discrete sections assembled together to form the layer. The sections may be elongated, lateral sections disposed around a circumference of the stem pipe, and oriented parallel to a longitudinal axis of the stem pipe. In addition, the sections may be annular, longitudinal sections disposed along a length of the stem pipe, and oriented perpendicular to a longitudinal axis of the stem pipe.
In addition, the sections may be disposed in rows oriented perpendicularly to a longitudinal axis of the stem pipe. The sections of each row may be offset with respect to the sections of an adjacent row. Alternatively, the sections may be disposed in columns oriented parallel to a longitudinal axis of the stem pipe. The sections of each row may be offset with respect to the sections of an adjacent column.
In accordance with another aspect of the present invention, the plurality of sections may include protrusions extending therefrom, and indentations extending therein. The protrusions and indentations of adjacent sections can be mated to maintain relative positioning between the sections.
A method for fabricating a composite buoyancy module includes the step of providing an elongated stem pipe which is configured to receive the riser therethrough. A layer of buoyant material is disposed about the stem pipe to form a mandrel. Resin impregnated fiber is wrapped around the mandrel to form a composite shell around the layer of buoyant material. Again, the layer of buoyant material may be formed by assembling a plurality of sections together around the stem pipe.
Additional features and advantages of the invention will be set forth in the detailed description which follows, taken in conjunction with the accompanying drawing, which together illustrate by way of example, the features of the invention.